There is no one that can answer this question for you. If I said, "I tried both and, in six months, I had $1,043,434 in USD donations and 1 Bitcoin", does that help? No, because it's "general information", not specific advice/data from the same audience as yours. If you're building a blog for people over 80 years old who want to check out their health insurance options in the US, obviously USD. If you're designing a tech review site for bleeding edge folks, maybe then BC. But unless you know your target audience's desires/comfort level, there's no way to answer this.

As others have said, offering both is a good idea. Unless you scare people by offering them choices they don't understand (many 80yo people don't understand BC and you might risk losing any donation from them if you paralyze them at the donation level).

Well if you are after regular Joe to donate then USD. Most people outside of tech still don't know about or understand bitcoins.

A/B test it and let the data decide

How about both?

I want to keep it classy and thought that having multiple ways of donating would seem a bit desperate, no?

A/B test, as previously said. But you may not have enough visitors at this stage. So put the 2 buttons, and in 3 months, remove on of them. Then you will have the best solution for your very site, and you will have a slick design.

I am exactly at the same stage of you for one of my site... I'll have a clear answer for you in 2 to 4 months :)

That sounds like a good plan :)

I wouldn't find that desperate at all. Just throw up a "Donate here" and that goes to another page with multiple options - so it doesn't seem cluttered. Can always throw some small icons under the initial "Donate here" to show what's supported if you'd like. Split test it.

"remove one* of them. ". And seeming a bit desperate shouldn't be that important. What is important is : #1 how great your site is valuable to your user : give them great content/tools/functionality #2 user experience and, and #3 monetization.